Based on the toxicological experimental method,the changes of microscopic structures and the number of immune cells of head-kidney of the Ricefield Eel(Monopterus albus)were investigated after the fish was exposed separately under different concentrations of mercury ions(Hg ²⁺)and the chromium ions(Cr⁶⁺)on day 1,2,4,8.The results showed that:in control,head-kidney parenchyma consisted of the lymphatic tissue,hematopoietic tissue and sinusoid,nephrons were absent.The lymphatic tissue was primary composed of lymphocytes which crowdedly arranged to the lymphocyte rope or gathered the group.The hematopoietic tissue mainly consisted of the red blood cells which crowdedly arranged to the cell rope or gathered the group.Lymphatic tissue and hematopoietic tissue respectively occupied the certain region or interval distributed,and separated by sinusoids.The less melano-macrophage that scattered between the lymphocytes loose arranged,not formed obvious spherical body.Comparing with the control,histological structure of fishes exposed to two kinds of heavy metal ions displayed similar changes.Along with increasing of the heavy metal ion concentration and lengthening of the treatment time,melano-macrophages gathered melano-macrophage centre,and gradually increased in the number and distribution,then reduced.Lymphatic tissue gradually loosed,the arrangement was sparse and chaotic.The lymphocytes border was unclear,the number gradually reduced,and showed retrogressive tendency.The red blood cells were massively damaged,sinusoid was expanded.The influence of Hg ²⁺ on histological structures of head-kidney of the Ricefield Eel was greater than Cr⁶⁺,and the pathological phenomenon was more obvious too.